Revert "Cleanup enableSyncDefaultUpdate flag (#26236)"

[kassens]

This reverts commit b2ae9dd.

While the feature flag is fully rolled out, these tests are also testing behavior set with an unstable flag on root, which for now we want to preserve.

Not sure if there's a better way then adding a dynamic feature flag to the www build?

[react-sizebot]

Comparing: 0700dd5...9302f25

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	162.95 kB	162.95 kB	=	51.54 kB	51.54 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	164.48 kB	164.48 kB	=	52.03 kB	52.04 kB
facebook-www/ReactDOM-prod.classic.js	+0.02%	553.05 kB	553.18 kB	+0.02%	98.24 kB	98.26 kB
facebook-www/ReactDOM-prod.modern.js	+0.02%	536.90 kB	537.02 kB	+0.03%	95.56 kB	95.59 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 9302f25

[rickhanlonii]

I think the "www" in www-dynamic is a misnomer. It's more like ReactFeatureFlags.variant.js. The flags actually used in www are in ReactFeatureFlags.www.js, and the www-dynamic are the flags used in the test-www --variant={variant} tests.

[acdlite]

It’s simpler than that, it’s just a mock of the ReactFeatureFlags module in www. You can reference __VARIANT__ anywhere; the reason to put them in that file is because it won’t get stripped out of the final build artifact.

The testing point is a bit moot currently though because we don’t actually run the build tests using the www config even though we should. But we still need the separation because that’s what causes Rollip/Closure to treat them as dynamic rather than static (and therefore eligible for dead code elimination) flags.

[kassens]

This didn't fully cleanly revert, but I did my best to restore the intent.

[kassens]

Hopefully, we can revert this again soon!